This invention relates to a pressure responsive control device for use in a compressor or the like to perform a control function in response to the occurrence of a selected fluid pressure. The invention relates more particularly to a sealed, pressure-responsive electrical switch which is manually resettable and trip-free.
In one known pressure-responsive switch, a diaphragm which is not hermetically sealed is spring mounted to move progressively in response to increase in fluid pressure applied to the diaphragm. A motion transfer pin transmits diaphragm movement to a switch arm which is initially disposed in one control position. The switch arm has a snap-acting portion adapted to trip and move the arm to a second control position when the motion transfer pin applies sufficient force to the arm. The diaphragm thus serves to move the switch arm to perform a control function in response to the occurrence of a selected fluid pressure. Spring-loaded push button means are arranged to apply a counteracting force to the switch arm when desired so that, after the applied fluid pressure has fallen below the selected level, the push button means can be selectively depressed to restore the switch arm to its first control position. The switch is thus manually resettable and an abnormal condition causing an overpressure condition or the like to occur can be corrected before the switch is reset. However, it is found that, in that known switch, a portion of the force applied by the push button means is transmitted to the diaphragm. As a result, the switch operation is not trip-free and, if the push button is held depressed, the switch is adapted to trip only at a different fluid pressure level.
Another well known pressure-responsive switch is characterized by its hermetically-sealed construction. In that switch, a switch arm is spring biased so that it is normally disposed in a first switch position. A diaphragm has a welded flange mounting the diaphragm in sealed relation in the switch and has a domed central portion which is movable from an original dished configuration to an inverted dished configuration with snap action when a selected fluid pressure is applied to the diaphragm. The diaphragm movement is transmitted to the switch arm through a motion transfer pin so that, when the diaphragm moves with snap action in response to application of the selected pressure, the switch arm is moved sharply to a tripped position. The diaphragm snaps back to its original dished configuration when the selected fluid pressure is removed permitting the switch arm to return automatically to its original position in response to the arm bias to reset the switch. For many applications it would be desirable if such a sealed switch could be manually resettable to permit correction of overpressure conditions or the like before resetting is permitted. It would also be desirable if such a sealed, manually resettable pressure switch could achieve trip-free operation.